The early promise
of smart structures  equipping spacecraft, aircraft, automobiles
and ships with networks of sensors and actuators that allow them
TO respond actively to changing environmental forces, an approach
predicted to revolutionize their design, construction and performance
 has never materialized.

That is because
researchers found that when such networks grew beyond a modest size
of about 100 nodes, they became too complex for central computers
to handle. In addition, the weight, power consumption and cost quickly
became prohibitive. In other words, they could not be scaled up
to large sizes.

Today, however,
recent advances in MEMS (micro-electromechanical systems) and distributed
computing appear to be overcoming these limitations, reported Kenneth
Frampton, assistant professor of mechanical engineering at Vanderbilt,
speaking at the Acoustical Society of America meeting in Pittsburgh
on June 6.

Frampton, who
is an expert in vibration and acoustics, and his colleagues have
incorporated these advances using a new approach, called embedded
systems, to design a smart vibration-reduction system for a 15-foot-long
rocket payload faring. Currently, the high noise and vibration levels
inside rockets when they are launched significantly increase the
cost of manufacturing satellites and other equipment boosted into
space. So a system that reduces these levels by even a small amount
would cut payload development costs substantially.

In the first
phase of the project, Frampton's group prepared and ran a detailed
computer simulation of the system that showed it should provide
a degree of vibration-reduction comparable to that of a centrally
controlled smart system.

"The most
important result of the simulation is that it shows that the embedded
system is scalable," says Frampton. "That means we should
be able to build it as big as we need to and it should continue
to function."

In the older
approach, all the sensors and actuators are connected to a central
computer. It receives information from all the sensors, processes
it and then sends instructions to all the actuators on how they
should respond. As the size of the structure and the number of sensors
and actuators increase, the amount of wiring required increases
dramatically. Difference in arrival times of information from the
nearest and farthest sensors also increases, as does the time it
takes the farthest sensors to receive their orders. The bigger the
system, the greater these and other problems become.

In an embedded
system, on the other hand, each node contains a PC-strength microprocessor
with a relatively simple program and modest amount of memory that
allows it to directly control the sensors and actuators wired to
its node. The microprocessor also communicates with its nearest
neighbors so they can work together. Depending on how the system
is set up, the processor also receives data from a certain number
of its nearest neighbors so that it can coordinate the actions of
its actuators with those of the other nodes. Although each processor
has considerably less capability than that of a central computer,
it has far less information to handle, and its workload does not
increase as the system gets bigger.

"Embedded
systems are also far more 'fault tolerant' than centrally controlled
systems," Frampton points out. If the central processor breaks
down, the entire system shuts down. But a decentralized system will
continue to work even when several microprocessors fail, although
probably with slightly diminished capability.

The second step
in Frampton's project is to put a 100-node system into an actual
rocket faring comparable to the simulated system. Then he will test
how well in it performs in the laboratory. This information will
allow the engineers to get better estimates not only of the system's
performance but also its weight and cost.

Collaborators
on the project include Research Assistant Professor Akos Ledeczi,
Associate Professor Gabor Karsai and Associate Professor Gautam
Biswas from the Vanderbilt Department of Electrical Engineering
and Computer Science.
The research is supported by the National Science Foundation, National
Aeronautics and Space Administration and Defense Advanced Research
Project Agency.